Another example of a cool thing to add would be something similar to Final Fantasy (sorry I'm a Square fanboy) IX's "Active Time Event" system, which allows you to watch your other characters if they're up to something while you're doing something.

For promoting character diversity:

1. In the final fantasy games (trend here) I really like the limit breaks that are unique to specific characters. Perhaps, have something like limit breaks but have character-dependent ways on how they work:

Thronk: Fills his limit meter when he takes damage, pulls off a powerful attack whose strength depends on stopping a moving bar at a certain spot (sort of like with Titus's limit break in FF-X or the power meter in golf games)

Gill: Fills his meter when he sees lots of stealable items, limit break consists of trying to steal multiple items from an enemy, sucess depends on hitting button combinations like in a fighter game or DDRR.

Valient: Fills his meter when enemies take damage, pulls off an attack like Thronk does but his attack depends on the outcome of a 1d20 dice roll.


So it would basically be:
Fill methods / limit outcomes / How it's calculated.

Create some way to really make these special to each different character and you might have something.

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Personally, I really liked the limit breaks for Tifa and Cait Sith in FFVII. Slot machines and dice have a unique element of risk to them thats really interesting in battle.

Tifa attack was calculated from X number of reels depending on if it was normal, critical, or miss (not sure... don't have the game anymore)

Cait did 100 x (his level/20)d6 dice damage. which was pretty neat.